The invention relates to flow generators for the supply of non-invasive positive pressure ventilation (NIPPV) for treating respiratory disorders, and in particular flow generator power supplies providing fast acceleration between treatment pressures.
Acute respiratory insufficiency may be treated with devices which provide positive pressure ventilatory assistance. Sleep disordered breathing, such as Obstructive Sleep Apnea (OSA) is also treated with these devices. A typical device comprises a controllable flow generator coupled to a nasal mask and provides a supply of breathable gas to a patient in the range 4 to 30 cmH2O positive pressure. Nasal prongs, a mouth mask or full face mask may be used as alternatives to a nasal mask. A reference to a mask herein is intended to include a reference to any of these patient interface devices.
The devices can supply gas at a relatively higher pressure during the inspiratory phase of respiration (IPAP) and a relatively lower pressure or atmospheric pressure during the expiratory phase of a respiration (EPAP). In other NIPPV modes, the pressure can be made to vary in a complex manner throughout the respiratory cycle. For example, the pressure at the mask during inspiration or expiration can be varied through the period of treatment.
Two ways of varying the pressure at the mask are (a) by mechanical or pneumatic valving (xe2x80x98valved machinesxe2x80x99) and (b) by utilising speed control of the motor/blower to control the output pressure of the flow generator (xe2x80x98variable speed machinesxe2x80x99). Valved machines are fast acting, however the valve mechanisms are complicated and expensive. Variable motor speed machines are considerably simpler, having only one moving part, namely the blower rotor. As such, these machines are cheaper to manufacture, and, in turn, cheaper to purchase.
Of particular interest is the time required for the transistion between EPAP and IPAP, this time being termed the xe2x80x98transition timexe2x80x99. In order that treatment efficacy and patient comfort are retained, the transistion time should be short. Variable speed machines generally have a longer transition time than valved machines. Transition times in the order of 200 to 500 ms are known. In the case of acute respiratory insufficiencies such as lung disease, emphysema and cystic fibrosis, a long transition time is unsatisfactory and can jeopardise treatment. For these conditions, a transition time of approximately 50 ms is generally acceptable.
To achieve a short transition time in a variable speed machine, the flow generator must accelerate quickly. Decelaration is readily achieved with excess energy lost to a heak sink or lost mechanically. Most currently used motor/blower units are capable of achieving the necessary acceleration if they are provided with sufficient power. The primary limitation is in the power supply. The power requirement during steady state typically is 20 Watts. However, for a short transition time, the power requirement rises to approximately 200 Watts. One possible solution is to increase the size of the power supply. The problem with that solution is that is also increases the weight and cost of the device.
It is an object of the invention to solve the problem of providing a flow generator with sufficient power to enable fast acceleration without the need for a physically large, complex and expensive power supply.
Accordingly, the invention discloses a power supply for a motor speed-controlled flow generator, comprising: a power supply circuit having input terminals to receive an input power supply and having output terminals to provide an output power supply for connection to an electrical load; and energy storage means coupled to said output, the energy storage means being charged by the output supply and operable to discharge the energy stored therein when the circuit cannot provide the full amount of a demanded load.
The invention further discloses a flow generator, including a blower for the provision of an output supply of air or breathable gas at a pressure elevated above atmospheric pressure, a motor driving the blower at a controlled speed, a motor controller for controlling the motor speed and hence the output pressure of the blower, and a power supply as defined immediately above coupled by the output terminals to the motor controller, the flow generator being operable such that on a demanded increase in treatment pressure from a first level to a second level, the energy storage means provides energy for acceleration that is not otherwise available from the power supply circuit of the power supply.
The invention yet further discloses motor speed controlled positive pressure ventilation apparatus, including a flow generator as defined immediately above, having connection to an air delivery conduit connected with a patient mask. In one preferred form, the apparatus provides bi- or multi-level CPAP treatment.
The invention yet further provides a method for supplying non-invasive positive pressure ventilation to a patient""s airways at a first pressure and a second, higher pressure, the method comprising the steps of:
providing a power supply with sufficient capacity to supply a variable speed motor-driven blower such that the first pressure can be achieved; and
providing a further static source of energy that has the capacity to replace the power supply and supply the blower such that the second pressure can be achieved.
It is particularly preferred that the energy storage means is a static capacitor connected across the output terminals. The energy storage means also can include a switch means, such as a transistor or a thyristor or a GTO device, to controllably switch the capacitor. Yet further, the circuit of the power supply can exhibit a voltage versus current characteristic having a first approximately constant voltage region up to a first current value and a second approximately constant current region beyond that value, in which second region the voltage reduces to a zero value. The capacitor will be switched, or come into operation at approximately said first current value.